The deposition of thin optical thin films is known in the art and several different methods or technologies have been used to deposit such films. Among the employed that have been applied, all of which are carried out in vacuum, are (1) Conventional Deposition (“CD”), (2) Ion Assisted Deposition (“IAD”), (3) Ion Beam Sputtering (“or IBS”), and (4) Plasma Ion Assisted Deposition (“PIAD”).
In the Conventional Deposition (CD) method, the material(s) to be deposited are heated to the molten state by either a resistance heating method or by electron bombardment, the heating being done in the presence of a substrate upon which a film is to be deposited. When molten, evaporation of the material occurs and a film is condensed on the surface of the substrate. At the molten material temperatures used by this method some disassociation of the evaporant takes place. While this dissociation is not a problem when an elemental material is being deposited (for example, elemental aluminum, silver, nickel, etc.), it does present a problem when the material to be deposited is a compound (for example, SiO2, HfO2). In the case of oxide materials, small amounts of oxygen are bled into the chamber during deposition in an attempt to re-store stoichiometry, a so-called reactive deposition. The films that are by the CD method are generally porous and lack sufficient kinetic energy (surface mobility) upon deposition to overcome surface energy (adhesion). Film growth is typically columnar (K. Gunther, Applied Optics, Vol. 23 (1984), pp. 3806-3816) with growth in the direction to the source and having a porosity that increases with increasing film thickness. In addition to high film porosity, other problems encountered with CD deposited films include index of refraction inhomogeneity, excessive top surface roughness, and weak absorption. Some improvements, though slight, are possible by adjusting the depositions rate and by increasing the substrate temperature during deposition. However, overall considerations of the final product dictate that CD techniques are not suitable for high quality optical components, for example, telecommunications elements, filters, laser components, and sensors.
Ion Assisted Deposition (IAD) is similar to the CD method described above, with the added feature that the film being deposited is bombarded with energetic ions of an inert gas (for example, argon) during the deposition process, plus some ionized oxygen (which in the case of oxide films is generally necessary to improve film stoichiometry). While ion energies are typically in the range 300 eV to 1000 eV, ion current at the substrate is low, typically a few micro-amps/cm2. (IAD is thus a high voltage, low current density process.) The bombardment serves to transfer momentum to the depositing film and to provide sufficient surface mobility so that surface energies are overcome and dense, smooth films are produced. The index inhomogeneity and transparency of the deposited films are also improved and little or no substrate heating is required for the IAD method.
Ion Beam Sputtering (IBS) is a method in which an energetic ion beam (for example, argon ions in the range 500 eV-1500 eV) is directed to a target material, typically an oxide material. The momentum transferred upon impact is sufficient to sputter-off target material to a substrate where it is deposited as a smooth, dense film. Sputtered material arrives at the substrate with high energy, perhaps several hundred electron volts leading to high packing density and smooth surface, but high absorption of the deposited films is a common by-product of the IAB process. As a result, an IBS process might also include an IAD source to both improve stoichiometry and absorption. While the IBS process is an improvement over CD and IAD, there are nonetheless problems with IBS. Such problems with the IBS deposition process include: (1) the deposition process is very slow; (2) it is more of a laboratory technique than a production process; (3) there are few IBS installations in existence, typically remnants from the telecom bubble and having one or two machines operated by a small staff; (4) substrate capacity is quite limited; (5) deposition uniformity over the substrate can become a limitation thus affect product quality and resulting in a high discard rate; (6) as the target is eroded the uniformity of the film being deposited changes, thus resulting in further quality problems and frequent target change-outs with associated down-time and costs; and (7) the bombardment energy is quite high, leading to disassociation of the deposited materials and hence absorption.
Plasma Ion Assisted Deposition (PIAD) is similar to the IAD process above, except momentum is transferred to the depositing film via a low voltage, but high current density plasma. Typical bias voltages are in the range 90-160 v and current densities in the range of milli-amps/cm2. While PIAD instruments are common in the precision optics industry and have been used to deposit films, there are some problems with the PIAD method, particularly in regard to the homogeneity of the deposited film. PIAD deposition has been described in commonly owned, copending U.S. application Ser. No. 11/510,140, Jue Wang et al inventors, published as US 2008/0050910 A1.
ArF excimer lasers have been is the illumination source of choice for the microlithography industry and have been used to mass-produce patterned silicon wafers in semiconductor manufacturing. As semiconductor processing has progressed from the 65 nm to the 45 nm node and beyond, microlithography technology has faced challenges in continuing to drive improved resolution, throughput, and stability. As a result, the expectations and requirements for excimer laser components have also increased. Alkaline earth metal fluoride optical crystals (CaF2, MgF2, etc.), especially CaF2, are the preferred optical material for making optical elements for ArF lasers due to their excellent optical properties and high bad gap energies. However, polished, but uncoated CaF2 surfaces degrade after only a few million pulses for fluences above ˜40 mJ/cm2 at 193 nm. Some solutions have been provided to extend the lifetimes of polished CaF2 components used in connection with excimer laser based systems. These include improved surface finishing quality and vacuum deposition of a thin layer of F-doped SiO2 as has been described in U.S. Pat. Nos. 7,242,843 and 7,128,984. However, the semiconductor industry constantly demands more performance from excimer laser sources, and as a result, excimer laser power and repetition rate over the last several years has been raised from 40 W to 90 W and from 2 KHz to 6 KHz, respectively. The laser power and repetition rate will be  further raised to 120 W and 8 KHz according to the technology roadmap of excimer lasers. These increases in power and repetition rate challenge the lifetime of existing laser optics components. As a result of these increases in power and repetition rate there is concern about premature failure of the optics due to bubble formation which has been observed in accelerated laser damage testing, for example using F—SiO2 coated CaF2 optical elements operating at the above higher power and reputation rates. For use in the higher power and repetition rate laser systems the environmental stability of F—SiO2 coated optical elements needs to be improved, especially in high humidity conditions.
Consequently, in view of the higher power and repetition rates that are becoming common in the industry there is a need for either a new process or improvements in the existing processes. In particular, there is a need for a process that can produce smooth and dense film coating on metal fluoride optical element and the elimination of bubble formation when such elements are used in the higher power and repetition rate laser systems.